54 Prof. W. F. Barrett on the Molecular Changes that 



is mounted on the supports S, S, passing through the helices 

 M, M', and abuts against the system of levers, which is shown 

 enlarged in fig. 2. A micrometer-screw N drives the bar into 

 close contact with the stud P (in fig. 2). The brass work sup- 

 porting N slides along the woodwork, and can be firmly clamped 

 to suit any length of the bar under experiment; the supports 

 for the bar and for the helices also slide, so that the current can 

 be sent round the bar at various positions. It was found neces- 

 sary to employ wedges within M, M', to keep the bar from the 

 lateral motion which occurred when the helices were animated. 

 The connexions with the commutator C and the battery are 

 evident. Fig. 2 shows the lever-frame with the front plate off. 

 D, D are the screws of the pillars which hold the frame together. 

 The first lever A is connected to the second B by the cross 

 piece C; to the second lever is attached the counterpoise 

 weight w, replacing the spring originally used. To the long 

 arm of B is fastened a silk thread, which passes round a grooved 

 wheel to the spiral brass spring s s ; the tension of the spring 

 can be regulated by turning the milled screw g ; by this means 

 the mirror m can be adjusted to any convenient position. The 

 vertical scale on which is cast the reflected ray r (fig. 1) is not 

 shown. 



But the difficulties in the way of making determinations 

 with this instrument were far more, than I had even anticipated. 

 The experience of the faults of one instrument led to the con- 

 struction of another, which, though yielding better results, would 

 at times perplex me by the most anomalous behaviour. On exa- 

 mination, these anomalies were traced to the hair spring which 

 held the mirror in position ; being of steel, it became magnetized 

 by the current and was displaced accordingly. This was removed 

 and a fine spiral copper spring substituted. The first results I 

 obtained with this improved apparatus showed that the cobalt 

 and iron behaved in a similar manner ; each displaced the light 

 in the direction of elongation 6*5 divisions of the scale on mag- 

 netization ; but with the nickel no action whatever was observed. 

 Increasing the battery power to twenty cells of Grove, a barely 

 discernible movement took place in the direction of elongation. 

 The very concordant results of two or three days' work seemed 

 satisfactory ; but unfortunately little reliance is to be placed in 

 them, owing to the fact that I discovered at their close a serious 

 source of error had been introduced through the inductive action 

 on an iron pin fastening the levers, by which a disturbance was 

 caused every time the current circulated round the helices. I 

 have had lately the apparatus entirely remade, and hope to work 

 at the subject again ; meanwhile I am endeavouring to procure 

 a purer specimen of cobalt*. 



* I thought it possible a lecture demonstration of the elongation of 



